1. Field of the Invention
The present invention relates to a piezoelectric device for converting an input mechanical energy into an output electrical energy, and vice versa.
More particularly, the present invention pertains to a piezoelectric device which comprises a thin ceramic substrate and a piezoelectric transducer formed on the substrate, wherein the transducer includes a piezoelectric layer arranged between two electrode layers.
2. Description of the Related Art
As generally known in the art, a piezoelectric device is typically used to convert an input mechanical stress into an output electric charge (voltage) and also to convert an input electric voltage into an output mechanical distortion. In the latter case, the device is sometimes called as an electrostrictive device. The term xe2x80x9cpiezoelectric devicexe2x80x9d as used herein is to be interpreted in its broadest sense, as encompassing an electrostrictive device as well. Similarly, the term xe2x80x9cpiezoelectric materialxe2x80x9d as used herein is to be interpreted in its broadest sense as encompassing an electrostrictive material as well. When the output of the piezoelectric device is a mechanical distortion, such distortion can be used to generate mechanical force, displacement or vibration.
Conventionally, piezoelectric devices have been used for various purposes. For example, in the field of optics or precision engineering, there is a progressive demand for actuators capable of adjusting lengths and/or positions of the optical path on a sub-micron order, or for sensors capable of detecting a fine mechanical displacement. There have been various proposals directed to such demands in conjunction with application of piezoelectric devices. Specifically, there is known a unimorph-type piezoelectric device which undergoes bending or flexural deformation upon application of an input electrical voltage, and which can be used for ink jet printing heads, acoustic radiators (i.e., loudspeakers and the like), vibrators, etc. The unimorph-type piezoelectric device can also be used to convert a bending or flexural deformation into an electrical voltage, e.g., as a microphone or sensor.
With reference, for example, to ink jet printing heads including a unimorph-type piezoelectric device, in view of customer""s or end users"" requirement for a high-quality and high-speed printing performance, it is desirable to realize an improved piezoelectric device which is of a minimized size and which operates at a low driving voltage and yet provides a satisfactory response characteristic. In the unimorph-type piezoelectric devices, it is highly desirable to reduce the thickness of the substrate so as to assure a sufficient amount of flexural displacement of the substrate or to enable generation of a sufficient potential level. On the other hand, however, a reduced thickness of the substrate often results in insufficient mechanical strength or rigidity of the substrate. Moreover, a known piezoelectric transducer formed on the substrate of the unimorph-type piezoelectric device is comprised of plate-like members, e.g., piezoelectric plate and electrode plates, which are typically bonded to the substrate by means of adhesives. Such a structure may give rise to disadvantages in that, due to deterioration with time of the adhesion strength, it becomes difficult to effectively transfer to the substrate a mechanical distortion which has been generated by the piezoelectric plate.
It is a primary object of the present invention to provide an improved piezoelectric device having a relatively high mechanical strength, which is capable of generating a relatively high degree of mechanical distortion upon application of a relatively low electrical voltage, which provides satisfactory reliability and response characteristic, and which is compact and thus suitable for a high density integration.
Another object of the present invention is to provide an improved piezoelectric device in which the piezoelectric transducer is maintained stably and firmly bonded to the substrate, thereby allowing a mechanical distortion to be effectively transferred from the transducer to the substrate.
According to the present invention, there is provided a piezoelectric device which comprises an at least locally thin-walled ceramic substrate and at least one piezoelectric transducer formed on the substrate. The transducer includes a piezoelectric layer arranged between two electrode layers. The transducer is in the form of a film which may be formed by a film-formation process known, per se. The film has a convex shape and protrudes toward the ceramic substrate.
With the above-mentioned arrangement of the present invention, the transducer having a convex shape protrudes toward the substrate. The protrusion is in a direction in which it is caused to displace during a normal operation of the device, thereby facilitating the displacement of the transducer even at a low input voltage level. Furthermore, the surface curvature of the transducer provides a relatively high mechanical strength or rigidity suppressing displacement of the transducer in a direction away from the substrate.
As will be fully described hereinafter, the convex shape of the transducer and a corresponding curvature of the substrate is advantageously formed during a heat treatment or sintering of the piezoelectric film layer, due to a predetermined relationship of the firing shrinkage and the coefficient of linear expansion of the piezoelectric film layer relative to the coefficient of linear expansion of the ceramic substrate. Thus, preferably, the ceramic substrate comprises a material having a coefficient of linear expansion within a range of 60xc3x9710xe2x88x927/xc2x0 C. to 120xc3x9710xe2x88x927/xc2x0 C.
Advantageously, the material for the ceramic substrate having the coefficient of linear expansion within the above-mentioned range comprises zirconium oxide as a main component, having a crystal phase that is completely or partially stabilized preferably by the addition of at least one member selected from a group consisting of yttrium oxide, cerium oxide, magnesium oxide and calcium oxide. Such a substrate material exhibits high mechanical strength, high fracture toughness and low chemical reactivity with the piezoelectric material during heat treatment, even when the substrate has a small thickness.
The average crystal particle size of the ceramic substrate is preferably 5 xcexcm or less, more preferably 1 xcexcm or less, in view of the desired mechanical strength of the substrate. The thickness of the thin-walled ceramic substrate as a whole, or the thin-walled region of the ceramic substrate is preferably 50 xcexcm or less, more preferably 30 xcexcm or less, and further more preferably 10 xcexcm or less. The thickness of the piezoelectric transducer is preferably 100 xcexcm or less, and more preferably 50 xcexcm or less.
Since, according to the present invention, the piezoelectric transducer is formed as a film on the thin-walled ceramic substrate, or the thin-walled region of the ceramic substrate, it is possible to generate a sufficient amount of displacement even at a relatively low operational voltage. At the same time, it is possible to achieve an excellent response characteristic and to generate a large force or high potential. Moreover, formation of the transducer by a film formation process makes it readily possible to provide the device with a number of piezoelectric transducers on the same substrate without using adhesives, thereby enabling a high density integration of the device.